In a nuclear magnetic resonance (NMR) spectrometer, a sample is positioned in an NMR probe that is located in a bore of a (typically superconducting) magnet and immersed in a high-strength (typically several Tesla) static magnetic field B0 generated by the magnet. In the magnet bore, the sample is surrounded by one or more sample coils. These coils apply a pulsed magnetic field B1 (typically orthogonal to the B0 field) oscillating in the radio frequency (RF) range (e.g., 40-900 MHz) to the sample. The coils are tuned to resonantly excite one or more desired types of NMR-active nuclei of the sample. The resonance condition is fulfilled if the frequency of the applied RF energy equals the resonance (or Larmor) frequency ν0 of the NMR-active nucleus being irradiated, which depends on the type of nucleus and the strength of the B0 field. At resonance, the B1 field efficiently transfers electromagnetic energy to the nucleus and causes a change in energy state. During the delay interval between pulses the nucleus emits an RF time-domain signal, known as a free-induction decay (FID), as a result of this perturbation. The FID decays in the interval as the excited nucleus relaxes back to its equilibrium state. The FID is picked up as an NMR response signal by the coil (the same coil utilized for excitation or a different coil). Electronics of the NMR spectrometer amplify and process the NMR response signal, including converting the signal from the time domain to frequency domain by Fourier transformation, to yield an NMR spectrum in the frequency domain. The spectrum consists of one or more peaks whose intensities represent the proportions of each frequency component detected. Thus, NMR spectra can provide useful information indicative of molecular structure, position, and abundance in chemical, biochemical, and biological species of interest.
The NMR probe includes the coil(s) and an NMR probe circuit. The NMR probe circuit provides RF communication between the coil(s) and the associated electronics of the NMR spectrometer (e.g., RF transmitting and receiving circuitry). The NMR probe circuit may be configured to provide more than one probe channel, with each probe channel being configured for resonantly exciting a different type of nucleus. In general, an NMR probe circuit serves to impedance-match the sample coil at one or more resonance frequencies, provide tuning in a narrow band near (below and above) these frequencies, and isolate different probe channels from one another. An NMR probe will usually have two sample coils that are coaxially nested around the sample. In one common configuration each sample coil is doubly resonated, resulting in a total of four resonances (channels) in the probe, while in other configurations more or less channels are possible. As noted above, each resonance corresponds to the Larmor frequency of a nucleus of interest contained in the sample. One of the channels, however, may be tuned for irradiating the deuterium (2H, or “D”) species of a “lock solvent” to produce a reference signal utilized for offsetting the drift of the B0 field (deuterium field-frequency locking, or deuterium locking). Environmental variables such as sample solvent dielectric, temperature, etc. can affect the tuning of the probe. Field strength will also vary slightly from one magnet to another, causing the same nuclei to resonate at slightly different frequencies in each magnet. For these reasons, the probe circuit must provide some means for adjusting (tuning) the sample coil's resonant frequency just prior to running an NMR experiment. This tuning is usually accomplished with variable capacitors.
In a conventional NMR probe circuit individual circuit components are soldered directly together, and hence are arranged in a three-dimensional free-form configuration. The free-form configuration has been considered to be advantageous because it generally minimizes inductance, stray capacitance and parasitic loss while maintaining the capacity for handling high voltages. While such a configuration can thus result in high performance, it does so at the cost of long design cycles (e.g., by prototyping), poor reproducibility, and expensive and time-consuming repair. Additionally, coupling between tightly packed circuits can lead to additional performance and design issues. These circuits must be carefully tuned and adjusted to operate properly.
In some NMR spectrometers, components of the probe (such as sample coils, circuit components, and/or preamplifiers) are cryogenically cooled (typically down to 20 K) via thermal conduction to a heat exchanger containing a cryogenic fluid such as liquid nitrogen or liquid/gaseous helium. The NMR sample, on the other hand, may be held at room temperature or other non-cryogenic temperature. Cryogenic probes reduce thermal noise by reducing the electrical resistance of the sample coils and thus may operate at high Q factors. NMR probe circuits having the conventional free-form architecture typically employ chip capacitors, which are prone to structural failure (and thus operational failure) at cryogenic temperatures.
In view of the foregoing, there is an ongoing need for NMR probe circuits that address the problems mentioned above.